Fly ash discharged in large amounts as a byproduct from a pulverized coal firing boiler of a thermal power plant, etc. is used in many fields, as a cement raw material, for example.
Fly ash usually contains unburnt carbon. It is known that if the unburnt carbon content is high, unburnt carbon-related problems may arise. For example, when fly ash is used as a cement admixture, high fly ash content may increase the usage of admixtures such as AE agent, cause black portions to appear on a hardened material, or physical properties of the hardened material may degrade. For these reasons, fly ash has not been used in large amounts.
To solve these problems, some methods of reforming fly ash by decreasing unburnt carbon contained in fly ash have been presented. For example, a method of separating unburnt carbon by floatation separation using the difference in wettability between unburnt carbon and ash content is known.
With the method and device for stabilizing coal ash for concrete disclosed in Patent Literature 1, a cylindrical slurry mixer shown in FIG. 3 of Patent Literature 1 is used. This slurry mixer includes a feed pipe for supplying coal ash, water filling pipe, bubble discharge pipe, air pressure-feed pipe, ash discharge pipe, pipe for discharging iron content, and agitation plate having perforated rotating plates and integrating an electrical magnet. Each pipe is equipped with a control valve.
In Patent Literature 1, water containing coal ash and a surface active agent is fed into the slurry mixer, and the mixture is agitated using the agitation plate while air is pressure-fed into the slurry mixer to generate bubbles. Carbon dioxide in air is used to decrease the basicity of highly alkaline fly ash, and iron content is separated using the magnet included in the agitation plate. While the above operations are performed, unburnt carbon is separated, entrained in bubbly ash discharged.
The unburnt carbon removing method disclosed in Patent Literature 2 adds water to fly ash to make a slurry mixture. Using the shearing force of agitation blades rotating at high speed in a surface modification device, activation energy is generated on the surface of unburnt carbon to allow the unburnt carbon to have lipophilic property, and make a collecting agent attach to the lipophilic unburnt carbon. The unburnt carbon is then made to attach to air bubbles by using a floatation machine to cause floatation separation to occur, and the unburnt carbon in fly ash can thus be separated.
The floatation machine shown in FIGS. 4 and 5 in Patent Literature 2 includes a plurality of chambers created by dividing a tank with partition walls, an agitator provided within each chamber, an external pipe including an air induction pipe and a hood installed around each agitator, a froth discharge path provided on both sides of the tank, and a plurality of waterwheel-shaped froth scrapers.
With this floatation machine, the slurry supplied through the slurry inlet on the upstream end face flows into the chambers separated with the partition walls. The slurry is agitated in each chamber by the agitators, and air is sucked in through the air induction pipe to generate air bubbles. Unburnt carbon attaching to these air bubbles goes up, is scraped to outside the tank with the froth scrapers, flows down to the froth discharge path, and then discharged to outside the machine through an aggregated froth path. The fly ash remaining within the tank is discharged with water to outside the machine as tails through a takeout port on the downstream end face.